Introduction

It is a well known fact that nuclear magnetic resonance (NMR) is commonly
used in well logging measurements and for routine laboratory core analysis.
However, many are not aware of the principle behind NMR and its advantages for
the core analyst. It is commonly used to determine porosity and pore size
distributions but it is important to note that NMR can also measure fluid
mobility parameters such as bound volume irreducible (BVI), free fluid index
(FFI), clay bound water (CBW) and effective porosity.

NMR can also easily and effectively measure permeability, capillary pressure,
and oil/water and gas/water contents. These parameters are measured with high
level of precision using comprehensive software, which is user-friendly and can
be easily operated by a novice in NMR. The technical details given below are
aimed at introducing NMR to the petrophysicist and core analyst who are not very
familiar with NMR.

Technical Background

When a sample is placed in a magnetic field and activated with a quick pulse
of radio frequency (RF), NMR signals are generated from liquids such as brine or
oil. An NMR signal is formed instantaneously after the pulse, which then dies
away with a characteristic decay rate or relaxation time known as T2.
The signal amplitude immediately after the pulse indicates the total amount of
fluid present. T2 of the signal provides important information about
the physical environment of the liquids.

In pores filled with a single fluid, there are two key components to the NMR
signal, one signal is generated from the fluid far from the pore walls and
another close to the pore walls. The nature of NMR signals in fluids far from
the pore walls is similar to those from bulk fluids having comparatively long
relaxation times, whereas fluids close to the pore walls undergo a process of
adsorption and desorption with the pore walls which has the effect of
drastically reducing their NMR relaxation times.

In large pores, the dominant effect is from the bulk fluids, so larger pores
have longer NMR relaxation times. In smaller pores, the surface-to-volume ratio
is much higher, hence the fluids near the pore wall dominate the NMR signal, and
smaller pores display overall shorter NMR relaxation times. This process is
illustrated in the figures below.

Fluids in large pores have long T2 decay times, like
bulk fluids

Signals from fluids in smaller pores are modified by surface
interactions

Of course, practically it may not feasible to take NMR measurements from
individual pores. The entire core must be measured at once, hence the resulting
NMR signal is a composite of all the NMR signals from the different pore sizes
in the core.

Mathematical Procedure - Inversion

Inversion is a mathematical procedure that involves processing the composite
NMR signal and separating it into its components. For every different pore size
in the core, there is one T2 component. Practically, the analysis is
restricted to a maximum of about 256 individual T2 components. A
T2 distribution is obtained which shows the relative population of
the individual T2 decay times that make up the composite NMR signal
from the core. Since long T2s come from large pores, and short
T2s from small pores, this T2 distribution can also be
considered as a model of the pore size distribution in the core.

Useful Measurements – Porosity, BVI, FFI, CBW and
Permeability

After obtaining the basic pore size distribution from the NMR data, a number
of helpful petrophysical parameters are conveniently and rapidly inferred. The
total porosity is the integral of all the T2s which is the area under the curve
when compared to the signal from a known reference.

In case the core is centrifuged the NMR measurement is repeated, the integral
of the second data set is the irreducible fluid (BVI), while the difference
between the two is the Free Fluid Index (FFI).

It is essential to note that the area under the saturated T2
distribution curve, which gives us a measure of total porosity, is influenced by
the curve shape. In case, the T2 curve does not represent all the
T2s present in the sample, then there will be some missing signals
from certain pores which will result in porosity being under reported. This is
possible if the instrument being used is not capable of detecting short
T2 signals from small pores, which would result in small pores not
being accounted for. The instrument being used must be able to perform sustained
measurements using short time-to-echo (TE) values.

This effect is shown in the following figure, which shows three T2
distributions obtained on the same core sample with TE=100μs (green),
200μs (red) and 600μs (blue).

The green distribution line at TE=100 μs and the red distribution
line at TE=200 μs are more detailed at the shorter T2
values or the smaller pores than the blue distribution line at TE=600
μs. The total integral signal recorded for the green distribution is
considerably higher corresponding to a higher porosity value. The results are
shown in the following table.

Sample

TE
(μs)

Acquisition
Time (min)

Number of
Scans

Signal to
Noise Ratio

NMR
Porosity (ml)

1-4R

100

2

80

220.18

4.248

200

5.5

224

203.06

2.175

600

21.5

864

200.16

1.151

It is usually assumed that all T2s of 2.5 ms or below come from
clay bound water, which are integrated separately to report CBW, and
thence Effective Porosity. Permeability can be determined from the relaxation
data using the Schlumberger or Coates model. The LithoMetrix software
incorporated in every GeoSpec2 instrument enables the measurements accurately and
quickly. An accessory known as pulsed field gradients (pfg) when added to the NMR instrument
enables measurements on flow, distribution or diffusion of fluids within core
sample.

The other measurements include the following:

Capillary pressure

Pore throat distribution

Wettability

Fluid typing

Capillary Pressure

The measurement of capillary pressure, which previously took several weeks to
months is now possible with NMR in hours or days with more number of data
points. The GeoSpec instrument has a patented GIT-CAP technique, which combines
1-D profiling and conventional centrifuge methods recording NMR signals in a
single dimension along the axis of the core. The method involves saturating a
core and obtaining a 1-D profile measurement through the NMR
instrument. An almost uniform fluid distribution is seen along the core
length as shown in the figure below.

The core is then spun in a standard centrifuge, which enables the fluid to be
shifted towards the core outer end and another 1-D profile with 30 – 40 points
is obtained.

This process is repeated at another centrifuge spin speed, after which the
data is analyzed. The calculation of capillary pressure is based on the
knowledge of the centrifuge spin speeds and the changes in the saturation
profile.

A number of factors need to be kept in mind, which are listed below:

It is possible to measure four times more samples than traditional methods
as only two centrifuge speeds are required compared to eight previously

For each saturation profile around 30 to 40 data points are obtained along
the sample as compared to one spin speed conventionally

A standard centrifuge without fluid collection measurement or stroboscope is
used.

Pore Throat Distribution and Wettability

The Washburn equations are used to determine pore throat distributions after
obtaining the Pc curve. The imbibition and secondary drainage curves generated
by the NMR Pc measurement enable prediction of USBM wettability. The
calculations are fed into the software and follow as a natural consequence of
the NMR Pc Data. An additional advantage is that the NMR measurement is
non-destructive and can use reservoir wettability and fluids.

Pore throat distribution obtained from Pc data

Fluid Typing

The GeoSpec2 instruments fitted with pulsed field gradients (pfg)
can perform measurements that are dependent on the flow, diffusion, or
distribution of the fluids within the core sample. One application of this is
for fluid typing.

Different fluids such as partially bound water and heavy oils may have
similar T2 values but they have different diffusion characteristics
hence both diffusion data and T2 need to be collected for accurate
separation of the fluid types. These measurements are possible with GIT system
software with a pfg accessory.

Conclusions

NMR is now a well-established core analysis tool, capable of making a wide
range of core measurements, from pore size distributions to capillary pressure,
on a single instrument such as the GeoSpec2. Oxford Instruments and Green Imaging Technologies have
partnered to enable an integrated hardware and software solution so that a wide
range of measurements can be performed with considerable ease, with no
specialized knowledge of NMR.

Today Oxford
Instruments Magnetic Resonance are focused on solving complex and often
unique technology problems for the understanding of biomolecular structure and
function across industrial, life science and drug discovery applications. In the
industrial process area, our low field benchtop instruments offer fast, accurate
and simple measurement solutions to routine Quality Control problems.

This information has been sourced, reviewed and adapted from
materials provided by Oxford Instruments Magnetic Resonance.